Welcome!
And LOTS more details. Tank size. How long has it been set up? Lighting. Amount of rock (live or otherwise - it all becomes live). Circulation - both in tank and return pump (assuming sump). Skimmer? You'll want one with eels - they tend to be very messy eaters Water parameters - please include Ammonia, Nitrite, Nitrate, specific gravity, and temp. With just eels and clowns , Calcium, Alkalinity, and Magnesium really don't matter at this point - any salt mix will have plenty of them.
What kind of Eels? Some will do ok with your clowns, some will eat them. Some stay (relatively) small. Others can hit 8'. They also tend to be very long lived. I had my 4' Gymnothorax Undulatus (Undulated Moray) for 25 years. He was the sole inhabitant in a 300g total volume system. (150DT, 120g refugium, 50g sump) His name was Hannibal. He ate EVERYTHING I tried to put in with him except a softball sized hermit crab.
What kind of clowns? Depending on type, it can make adding other fish in the future difficult due to their territoriality, especially in smaller tanks.
IMHO, carbon is almost always a good thing to use. I run it 24/7/365 and have for nearly 30 years.
The more pertinent information you can give us the more we will be able to help
hth